As a filter for exhaust gas, a diesel particulate filter (DPF), for example, is incorporated in, for example, the exhaust gas system of diesel engine in order to capture and remove the particulate contained in the exhaust gas emitted from the diesel engine or the like. Filters for exhaust gas such as the above DPF and the like are constituted by a honeycomb structure wherein a plurality of honeycomb segments are bonded integrally by a bonding material.
Such a honeycomb structure has a construction wherein a plurality of cells each functioning as a passage of a fluid, surrounded by porous partition walls made of a silicon carbide, etc. are arranged so as to be parallel to each other in the central axis direction of the honeycomb structure. In producing the honeycomb structure, a plurality of honeycomb segments having such a shape that each segment is part of the honeycomb structure and, when bonded to each other in a direction normal to the central axis of the honeycomb structure, forms the honeycomb structure, are bonded integrally by a bonding material to obtain a bonded body having an intended sectional shape (e.g. a circle) when cut by a plane normal to the central axis direction of the bonded body; then, the outer surface of the bonded body is coated with a coating material. At each end face of the honeycomb structure, the ends of cells adjacent to each other are plugged alternately. That is, one cell is open at one end face and plugged at other end face, and any other cell adjacent thereto is plugged at the same one end face and open at the same other end face.
By producing the honeycomb structure in the above constitution, an exhaust gas can be taken into the honeycomb structure from each one end of given cells, i.e. gas-incoming cells, be moved into cells adjacent to the gas-incoming cells, i.e. gas-leaving cells, via porous partition walls, and be discharged out of the honeycomb structure from the gas-leaving cells; during the movement of the exhaust gas through the partition walls, the particulate present in the exhaust gas can be captured by the partition walls and thereby the exhaust gas can be purified.
The bonding material for bonding a plurality of honeycomb segments integrally and the coating material for coating the outer surface of the resulting bonded body, both used in production of the honeycomb structure are required to have good coatability. In particular, the bonding material is required to also show good spreadability in press-bonding of honeycomb segments. In order for the bonding material and the coating material to have such properties, it is effective to lower their viscosities when they re coated. However, a low-viscosity bonding material and a low-viscosity coating material need to contain a large amount of a solvent, making large the shrinkage caused by solvent removal during drying. As a result, after bonding with the bonding material, drawbacks such as peeling of bonded area, cracking and the like have tended to generate; and, after coating with the coating material, drawbacks such as small holes, cracking, fretting and the like have tended to generate.
As a countermeasure for the above problem, there was disclosed (see a patent literature 1) a ceramic structure wherein an organic binder is added to a bonding material in order to suppress the migration taking place during drying and curing, consequently suppress the above-mentioned drawbacks, and provide a honeycomb structure of improved durability.
In the patent literature 1, as preferred examples of the organic binder, there are mentioned polyvinyl alcohol, methyl cellulose, ethyl cellulose and carboxymethyl cellulose. In the literature, it is disclosed that of these organic binders, carboxymethyl cellulose is preferred because it can ensure fluidity of bonding material during bonding.
Patent literature 1: JP-B-3121497